1. Field of the Invention
The present invention relates to a display apparatus. More specifically, the invention relates to a method of controlling a driving pulse that drives a display apparatus formed of light emitting diodes (LEDs).
2. Description of the Related Art
Generally, light emitting diodes consume less power, emit less heat, are small and light, and can be driven by low-voltage direct current power. Light emitting diodes are typically used as pilot lamps, in other words, indicators, in various electronic/electric and industrial products.
Recently, light emitting diodes have attained a level of brightness that they are now widely used as warning lights on roads or the like, and are used as the light source of display devices such as light panels, subway guide electric signboards, and public relations and advertisement signboards. Light emitting diodes having light directionality can be used as a light source in a transparent liquid crystal display (LCD), in other words, a display apparatus.
A light emitting diode (hereinafter “LED”) utilizes the light generated when a hole and an electron recombine at the junction of p-type and n-type semiconductor materials, and have a much faster response time than a filament-type lamp. However, as an LED switches on and off it flashes at full brightness immediately after switching on, and then light disappears instantly when the LED is switched off.
Generally, direct current constant voltage is used as the input of a driving circuit for lighting an LED, and the brightness of an LED can be controlled by controlling the voltage intensity or current. LED switching is controlled through the ON/OFF control of direct current voltage, or the ON/OFF control of the current path of an LED.
When driving an LED corresponding to a pixel in a display area, an LED driving signal is controlled by a driving waveform, as shown in FIG. 1. The driving waveform is generated by an LED driving signal generator and supplied to a current converter, which converts the driving signal into current, thus controlling LED switching. FIGS. 1(a), (b), and (c) illustrate three driving signals for driving RGB LEDs. If the ON-signal current detected by a current detector is more than a predetermined current level, a current controller will decrease the current; if the detected current is less than a predetermined current level, the current controller will increase the current, thus enabling the LED to maintain constant brightness.
As shown in FIG. 1, the driving signal generated by the LED driving signal generator is similar to an ideal pulse (e.g. has a predetermined pulse width ‘P’ as indicated in FIG. 1(c)). However, the signal that passes through a voltage-current converter and an LED experiences a transition time, rising time, overshoot, and fall time, as shown in FIG. 1(d).
Overshoot occurs when a signal is converted into a pulse. Overshoot generates signal distortion and causes a time period to occur in which each LED is simultaneously turned off. Thus, signals distorted by overshoot cannot effectively be used to drive LEDs.
FIG. 2 illustrates three RGB driving waveforms on the same line consecutively, demarcated as shown in FIG. 2(a).
The general response characteristics of an LED are such that the rise time is longer than the fall time. Also, LEDs require a certain voltage threshold before they turn on., shown in FIG. 2(b) as “th”. Accordingly, there is a period during which each RGB LED is turned off, shown in FIG. 2(b) as “y”. In addition, overshoot is generated when a waveform is converted into a pulse, shown in FIG. 2(b) as “x”. Signal distortion occurs if the signal is displayed during this overshoot period, so the overshoot period is not used.
Considering that technology for improving light-collecting efficiency and brightness of a light source is extremely important in a display apparatus, conventional LED driving methods, as explained above, fail to maximize the efficiency of an LED.